A common method for treating stenosed vessels or other blocked passageways is to utilize an expandable prosthesis or stent. The stent is delivered to a target site, expanded, and affixed in place. The deployed and affixed stent creates an enlarged lumen space for passage of blood and also provides increased reinforcement of the vessel wall, in order to maintain the cleared passageway. Stents can be covered or uncovered. Some existing covered stents provide a series of interconnected metal rings encapsulated by a layer of biocompatible material. In uncovered stents (referred to as “bare metal stents”), the series of interconnected metal rings are exposed directly to the vessel and do not include any sort of cover.
Covered stents are desirable over bare metal stents in many circumstances as they provide more uniform coverage (e.g., gaps between stent struts are not exposed to walls of the target site, etc.) and improved luminal patency over bare metal stents in addition to other benefits. However, covered stents suffer from some operational shortcomings like foreshortening and recoil. In contrast, many bare metal stents exhibit little to no foreshortening and minimal recoil when deployed. Yet, the very same stents often will foreshorten by up to twenty five percent or more when deployed with a cover. However, in some instances, covered stents outperform bare metal stents in freedom from restenosis within particular defined periods of time (e.g., during the first 18 months following stent implantation).
For example, FIG. 1 depicts one example of a known stent design for a conventional covered stent 110 (cover not shown for clarity). The stent 110 is depicted in a flattened, discontinuous state for purposes of illustration. The stent 110 includes a series of rings 112, which in actuality are generally cylindrical and continuous at their ends (i.e., connected at the tops and bottoms). The rings 112 are generally cylindrical and aligned in a series. Each ring 112 is generally made up of a plurality of repeating peaks and valleys 114a, 114b. In the example stent 110 of FIG. 1, each particular ring 112 is a mirror image of any ring 112 adjacent to it in the series. Accordingly, for any two adjacent rings 112, the peaks 114a of one ring 112 are circumferentially aligned with the valleys 114b of the other ring 112, and vice versa. Each iteration of a peak 114a and a valley 114b forms a repeating bent segment 122. The rings 112 fixedly secured to one another by a plurality of interconnecting members 116. Each interconnecting member 116 includes two elbows 118 and an elongate portion 120 disposed therebetween.
In general, each interconnecting member 116 can occupy any one of a number of different configurations. In an “up-up” or “down-down” configuration, the elbows 118 of a single interconnecting member 116 extend to the same side (e.g., to the top or the bottom, respectively) of the elongate portion 120. Said differently, in an “up-up” confirmation or a “down-down” configuration, the elbows 118 of a single interconnecting member 116 extend circumferentially in the same direction (e.g., clockwise or counterclockwise). On the other hand, in an “up-down” or “down-up” configuration, the two elbows 118 of a single interconnecting member 116 extend to different sides (e.g., to the top and the bottom, or to the bottom and the top, respectively) of the elongate portion 120.
In the example of FIG. 1, each of the interconnecting members 116 is either in a “down-up” configuration or an “up-down” configuration. This is a conventional and prevailing design for many known stents. Additionally, in the example stent 110 of FIG. 1, there are half as many interconnecting members 116 disposed between any two adjacent rings 112 as there are repeating bent segments 122 in any single ring 112. This too is a common design feature for known stents. The stent 110 includes a cover (not shown, for clarity). The cover can be constructed from a biocompatible material (not shown) such as expanded polytetrafluoroethylene (ePFTE).
As described above, the conventional stent 110 is often associated with operational drawbacks, including foreshortening and recoil, especially when covered. Foreshortening is the property of a stent decreasing in length during expansion. Recoil is the property of a stent contracting from an expanded diameter to a partially expanded or non-expanded diameter. Foreshortening is particularly problematic, as it compromises the ability of the stent 110 to ensure correct placement and treatment of the stenosed vessel, and it also increases risk of damage to surrounding intima during expansion. Furthermore, recoil is associated with other risks, such as reduced reinforcement, which can lead to greater occlusion and blockage of the affected passageway. In general, poor efficacy of covered stents (e.g., as caused by substantial quantity of foreshortening and substantial quantity of recoil) has been proven in clinical settings to be linked with higher risk of restenosis. Thus, for a stent to be operable and safe for use, these negative characteristics must be eliminated, or at least maintained at acceptably low levels.